The invention relates to a solar radiation concentrating system for use, for example, in a solar energy plant associated with a high-power solar energy receiver.
A. Rabl, xe2x80x9cTechnical Note. Tower reflector for solar power plantxe2x80x9d, Solar Energy, Vol. 18, pp. 269-271, 1976) is one of the earliest publications disclosing a solar radiation concentrating system of the above kind. The system comprises a primary concentrator in the form of a heliostats field installed on the ground plane, a tower reflector mounted on a solar tower above the ground plane and a secondary concentrator associated with a solar receiver disposed adjacent the ground plane.
As acknowledged in the Rabl""s disclosure, there exists a serious problem connected with a necessity to avoid overheating of the tower reflector which is to be exposed to the concentrated solar light of 100 suns or even more. With an ordinary construction of the reflector, based on metallic layers, the reflector requiring an intensive cooling thereof, which is quite difficult and onerous at high altitudes of the tower, would have absorbed a significant amount of this energy. In order to solve the overheating problem, Rabl suggests that the elements of the tower Fresnel reflector be in the form of rectangular prisms with total internal reflection.
U.S. Pat. No. 5,578,140 considers different disadvantages of the construction proposed by Rabl, which render it practically inapplicable, and suggests the provision of the tower reflector in the form of a sophisticated dielectric structure comprising a transparent substrate coated with a plurality of thin layers made of dielectric materials constituting highly reflective interference coating. The beam-splitting ability of the dielectric mirror makes its absorption coefficient negligible, whereby a necessity of cooling arrangements is eliminated. However, this solution is rather expensive.
It is the object of the present invention to provide a new solar radiation concentration system having advantages in the above respect.
In accordance with the present invention there is provided a solar radiation concentrating system comprising at least two reflectors successively arranged along an optical path of the system so that a first of said two reflectors reflects the radiation towards a second of said two reflectors, each reflector being capable of reflecting the radiation in a reflection range of wavelengths and absorbing the radiation in a absorption range of wavelengths, wherein the absorption range of wavelengths of said first reflector substantially includes the absorption range of wavelengths of said second reflector.
Thus, in system of the present invention the reflectors are chosen, according to their spectral characteristics, in such a manner that most of the reflection losses in the system will occur in the first reflector, which absorbs a significant fraction of the solar radiation within the absorption range of the second reflector, whereby the radiation which may be absorbed by the second reflector is practically excluded, or at least significantly reduced, in the radiation incident on the second reflector. The present invention takes advantage of the fact that glasses and metal coatings, of which reflectors of the kind used in the system of the present invention are usually made, have reflection and absorption properties depending on the spectrum of radiation incident thereon. Thus, for example, it is well known that Silver has high reflectivity at long wavelengths and significant absorption in the near UV range of wavelengths and Gold has absorption shifted to longer wavelengths. Also different glasses have different spectral behavior so that may options exist for the selection of spectral properties of the reflectors.
The idea of the present invention is contrary to the common practice according to which attempts have always been made to choose all reflectors with the highest possible reflection and minimal possible absorption. In the present invention, the advantage is being taken of the spectral behavior of the reflectors in order to control both the reflection and the absorption of radiation at each reflection stage of the system.
The solar energy concentrating system according to the present invention may be used in any system where successive concentrating devices are used, with the primary concentration being usually performed by a tracking mirror or lens and the final concentration being most often non-imaging. More complicated designs of the system according to the present invention ray include one or more intermediate reflectors as, for example, disclosed in the prior art publications considered above. In relatively small solar radiation concentrating systems, the primary concentrator may be in the form of a solar dish or trough, whilst in large scale solar energy plants, the primary concentrator is in the form of a heliostats field.
The solar radiation concentrating system of the present invention is specifically advantageous for use in a large-scale solar energy plant having a heliostats field, a tower reflector and, preferably, a non-imaging concentrator associated with a solar receiver. Since the heliostats are exposed to non-concentrated solar radiation, they may easily withstand the excessive absorption of radiation without the risk of being destructed, whilst the tower reflector, in which the overheating problems are now being solved by the reduced absorption, may have any appropriate design.
The solar energy concentration system according to the present invention may have only two reflectors or rather may be based on several successive reflectors so that the spectral characteristics of each reflector are used to control thermal conditions of its successor, i.e. the reflectors may have absorption which is being successively reduced, and the reflection which is being successively increased, from one reflector to another along the optical path of the system, the reflection and absorption ranges of wavelengths of the reflectors being correlated accordingly.